After its initial introduction, many people have asked “why start a new
library instead of contributing to an existing library?” (like Apache’s
libstdcxx, GNU’s libstdc++, STLport, etc). There are many contributing
reasons, but some of the major ones are:

From years of experience (including having implemented the standard
library before), we’ve learned many things about implementing
the standard containers which require ABI breakage and fundamental changes
to how they are implemented. For example, it is generally accepted that
building std::string using the “short string optimization” instead of
using Copy On Write (COW) is a superior approach for multicore
machines (particularly in C++11, which has rvalue references). Breaking
ABI compatibility with old versions of the library was
determined to be critical to achieving the performance goals of
libc++.

Mainline libstdc++ has switched to GPL3, a license which the developers
of libc++ cannot use. libstdc++ 4.2 (the last GPL2 version) could be
independently extended to support C++11, but this would be a fork of the
codebase (which is often seen as worse for a project than starting a new
independent one). Another problem with libstdc++ is that it is tightly
integrated with G++ development, tending to be tied fairly closely to the
matching version of G++.

STLport and the Apache libstdcxx library are two other popular
candidates, but both lack C++11 support. Our experience (and the
experience of libstdc++ developers) is that adding support for C++11 (in
particular rvalue references and move-only types) requires changes to
almost every class and function, essentially amounting to a rewrite.
Faced with a rewrite, we decided to start from scratch and evaluate every
design decision from first principles based on experience.
Further, both projects are apparently abandoned: STLport 5.2.1 was
released in Oct‘08, and STDCXX 4.2.1 in May‘08.